corrosion and its control -...

Corrosion and its Control CY 101

(Chemistry)

Instructor: Dr. Priyabrat Dash (Email: [email protected], Ph: 8895121141)

(Office: Biomedical 406) Webpage: http://homepage.usask.ca/~prd822/

June 11-15, 2013 National Institute of Technology,

Rourkela

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CY 101 (Corrosion and its Control)

Dr. Priyabrat Dash

NIT Rourkela, India

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Outline

• Class 1-3: Introduction to Corrosion,, Types of Corrosion, and Mechanism of Corrosion

• Class 4: Principle of Corrosion Control

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Dr. Priyabrat Dash

NIT Rourkela, India 2

What is Corrosion? Corrodere (Latin) – To eat away

• Corrosion may be defined as the gradual destruction of metals by the chemical or electrochemical reaction with the environment. During corrosion, the metals are converted to their metallic compounds at the surface.

• The loss of materials due to corrosion has become a great problem. The most common example for corrosion is the rusting of iron when it is exposed to atmospheric conditions. The rusting is due to the formation of hydrated ferric oxide on the surface. Another example is the formation of green film of basic copper carbonate on the surface of Cu, when exposed to moist air containing CO2.

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Environments in corrosion

Corrosion case studies

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1988

19-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

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Bhopal Gas Leak

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In the early morning of December 3, 1984, water inadvertently entered the methylisocyanate storage tank, where >40 metric tons of methylisocyanate were being stored. The addition of water to the tank caused a runaway chemical reaction, resulting in a rapid rise in pressure and temperature. The heat generated by the reaction, the presence of higher than normal concentrations of chloroform, and the presence of an iron catalyst, produced by the corrosion of the stainless steel tank wall, resulted in a reaction of such momentum that gases formed could not be contained by safety systems.

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Losses due to Corrosion

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Loss of metal by corrosion is a waste not only of the metal, but also of the energy, the water, and the human effort that was used to produce and fabricate the metal structures in the first place. In addition, rebuilding corroded equipment requires further investment of all these resources —metal, energy, water, and human.

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Direct losses include the costs of replacing corroded structures and machinery or their components, such as condenser tubes, mufflers, pipelines, and metal roofing, including necessary labor. Other examples are (a) repainting structures where prevention of rusting is the prime objective and (b) the capital costs plus maintenance of cathodic protection systems for underground pipelines. Direct losses include the extra cost of using corrosion - resistant metals and alloys instead of carbon steel where the latter has adequate mechanical properties but not sufficient corrosion resistance; there are also the costs of galvanizing or nickel plating of steel, of adding corrosion inhibitors to water, and of dehumidifying storage rooms for metal equipment. Indirect losses are more difficult to assess.

Losses due to Corrosion

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Why study Corrosion?

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1. Materials are Precious resources

2. Engineering design is incomplete without knowledge of corrosion

3. Applying knowledge of corrosion protection can minimize disasters

4. Corrosion- may contaminate stored food, dairy products , etc

5. Corrosion products cause pollution

6. Artificial implants for the human body ?

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Types of Corrosion

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1. Direct Chemical Corrosion or Dry Corrosion This type of corrosion occurs mainly through the direct chemical action of atmospheric gases such as O2, halogens, H2S, CO2, SO2, N2, H2 or liquid metals on metal surface in the absence of moisture.

2. Electrochemical Corrosion or Wet Corrosion This type of corrosion occurs when : a. A metal is in contact with a conducting liquid b.Two dissimilar metals or alloys are immersed partially in a conducting solution. This corrosion is due to the existence of separate anodic and cathodic areas between which current flows through the conducting solution.

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Direct Chemical Corrosion or Dry

Corrosion

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and other gases

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Direct Chemical Corrosion or Dry Corrosion Type 1: Oxidation corrosion

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It is brought about by the direct action of O2 present in the atmosphere on metals at low or medium temperature in the absence of moisture to form metallic oxides. Alkali and alkaline earth metals are rapidly oxidised even at low temperature. At high temperature almost all metals except (Ag, Au, Pt, Pd) are attacked.

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Direct Chemical Corrosion or Dry Corrosion Type 2: Corrosion by other gases

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If the film formed is protective or non-porous

(AgCl film, resulting from the attack of Cl2 on Ag)

If the film formed is non-protective or porous (dry Cl2 gas on Sn forming

volatile SnCl4)

The intensity or extent of attack decreases

The intensity or extent of attack increases

Direct Chemical Corrosion or Dry Corrosion Type 2: Corrosion by other gases

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Direct Chemical Corrosion or Dry Corrosion Type 3: Liquid Metal Corrosion

It is due to the action of liquid metal at high temperature on solid metal or alloy. This type of corrosion is found in nuclear power plants. Corrosion takes place either due to the dissolution of solid metal by the liquid metal or due to the penetration of liquid metal in to the solid metal.

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According to electrochemical theory, corrosion of metals occurs due to the following changes, when they are exposed to the environment. 1) A large number of minute galvanic cells are formed which acts as anodic and cathodic areas. 2) At anode the metal undergoes oxidation and electrons are liberated which migrates towards cathodic region 3) Oxygen of the atmosphere undergoes reduction at cathodic area in the presence of moisture forming hydroxyl ions at the cathode.

Electrochemical Corrosion or Wet Corrosion

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Explain the electrochemical theory of corrosion?

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Types of Electrochemical Corrosion

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1. Galvanic corrosion or Bimetallic corrosion When two dissimilar metals or alloys are electrically connected and exposed to an electrolyte, the metal higher in electrochemical series under goes corrosion. Thus when Zn and Cu are connected, Zn being higher in the series act as anode and undergoes corrosion and Cu which is lower in the series act as cathode and gets protected.

Ex: Steel pipe connected to copper plumbing and Lead – antimony solder around copper wire.

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What is Galvanic series?

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The arrangement of elements in the order of their standard reduction potential is referred to as emf or electrochemical series. Such a arrangement of few elements given in the table.

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1) A negative value indicates oxidation tendency and a positive value indicates reduction tendency with respect to hydrogen. 2) The metal with lower electrode potential are more reactive and as the electrode potential increases, the reactivity decreases and metals with higher electrode potentials are more noble. 3) Metals with lower electrode potentials have the tendency to replace metals with higher electrode potential from their solutions for example, Zn displaces Cu, Cu displaces Ag. 4) Metals with negative electrode potentials can liberate hydrogen from acidic solutions.

What is Galvanic series?

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Pitting Corrosion

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Pitting corrosion is a localized accelerated attack resulting in the formation of pin holes, pits and cavities on the metal surface. It is due to the breakdown or cracking of the protective film on the metal at specified points. This gives rise to the formation of small anodic and large cathodic areas. Once a small pit is formed the rate of corrosion will be increased.

Presence of external impurities like sand, dust, water drops etc on the surface of the metal can also be a cause for this type of corrosion. In this case, the small part below the impurity acts as the anodic area while the rest of the metal acts as the cathode area. Due to corrosion a small pit is formed at the anodic area which grows gradually.

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Concentration Cell Corrosion

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This type of corrosion is due to electrochemical attack on the metal surface exposed to an electrolyte of varying concentrations. Metal in contact with lower concentration will act as anode and undergoes corrosion. It is observed in chemical plants and storage tanks and also in marine structures like ships. Differential aeration corrosion is the most important.

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Differential aeration Corrosion

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This type of corrosion occurs when one part of the metal is exposed to a different air concentration from the other part. This causes a difference in potential between differently aerated areas. It is found that less oxygenated part acts as anode and more oxygenated part acts as cathode. Corrosion of metals partially immersed in a solution is due to differential aeration corrosion. Consider a Mg rod partially immersed in NaCl solution. The areas of Mg rod above and just below the water line are more oxygenated and will act as cathode. The remaining part of the rod which is well under the solution is less oxygenated and act as anode and undergone corrosion.

At anode

Water Line Corrosion

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It is another type of differential aeration corrosion. Corrosion in storage tanks, H2O tanks, marine structures etc is called water line corrosion. When water is stored in an iron tank, it is found that maximum corrosion occurs along a line just below the water level. The area above the H2O line is highly oxygenated and acts as cathode while the area just below the H2O line is less oxygenated and acts as anode and undergoes corrosion.

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Factors Affecting Corrosion

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The rate and extent of corrosion depends mainly upon two factors– 1. Nature of the metal or metallic conditions 2. Nature of the environment

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Nature of the metal or metallic conditions

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a. Position in Galvanic Series Metal higher in the galvanic series are more likely to undergo corrosion. b. Relative anodic and cathodic areas Corrosion is more rapid if the anodic area is small because there is more demand for electrons by the larger cathodic area. c. Purity of the metal Impurities in a metal form minute electrochemical cells and the anodic part gets corroded.

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Nature of the metal or metallic conditions

d. Physical state of metal The rate of corrosion is influenced by the physical state of the metal such as size, orientation of crystals, stress etc. The smaller the size of metal greater will be the corrosion and the stressed port of the metal also undergo more corrosion. e. Nature of corrosion product Metals like Fe, Mg etc form a non protective porous oxide film which causes maximum corrosion while metals like Al, Cr, Ni etc forms protective oxides . f. Solubility of corrosion product In electrochemical corrosion, if the corrosion product is soluble in the medium, then the corrosion proceeds at a faster rate. g. Volatility of corrosion product If the corrosion product is volatile rapid and continuous corrosion occurs.

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Nature of Environment

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a. Temperature Rise of temperature increases the rate of corrosion. b. Humidity Humidity air is directly related to the rate of corrosion. In humid condition atmospheric gases easily form electrochemical cell by which corrosion occurs to a great extent. c. Effect of pH Generally acidic media is pH<7 is more corrosive than alkaline and neutral media. d. Formation of O2 concentration cell If there is a difference in O2 concn around the metal, then the less oxygenated metal part becomes anode and the more oxygenated part becomes cathode and an O2 concentration cell is set up resulting corrosion.

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Nature of Environment e. Nature of ions Presence of anions like silicate ions in the medium leads to the formation of insoluble reaction products which inhibit further corrosion. On the other hand Cl- ions and NH4

+ ions etc destroy the protective surface film thereby exposing fresh metal surface for corrosion. Rapid corrosion of Al in sea water is an example. f. Presence of suspended particulars or compounds Particulars like NaCl, (NH4)2 SO4 etc along with moisture act as powerful electrolyte and promote corrosion. g. Conductance of the corroding medium In the case of underground and submerged structures, the conductance of the medium influences the rate of corrosion. Conductance of dry sandy soil is lower than that of clayey and mineralized soil. Hence the corrosion rate of metallic structures in lower in dry sandy soil than in clayey and mineralized soil.

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Distribution of disciplines in which active corrosion engineers have graduated

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� Ensuring maximum life of new equipment

� Preservation of existing equipment

� Protecting or improving the quality of a product in order to maintain or improve a competitive position.

� Avoiding costly interruptions of production.

� Reducing or eliminating losses of valuable products by spillage or leaks.

� Refitting of equipment withdrawn from service because of corrosion.

� Reducing hazards to life and property that might be associated with corrosion:

� Explosions of pressure vessels or piping systems

� release of poisonous or explosive gases or vapors

are a few examples.

So ..What would be expected from a corrosion engineer?

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The important methods used to control corrosion are, 1. Design and selection of the materials.

2. Protective coatings. a) Organic coatings – paints and enamels. b) Inorganic coatings i) Metal coatings – anodic and cathodic ii) Surface conversion coatings – anodizing, phosphating. 3) Corrosion inhibitors. 4) Cathodic protection. 5) Anodic protection.

Corrosion Control or Protection from Corrosion

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Inorganic Coatings

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i) Metal coatings – anodic and Cathodic ii) Surface conversion coatings – anodizing, phosphating. i) Metal coatings Deposition of protective metal over the surface of base metal(metal to be procted from corrosion) is known as metallic coatings. It is divided into Anodic and Cathodic metal coatings. a) Anodic metal coatings: Anodic metal coatings involve coating the base metal with more active metals, which are anodic to the base metal. Example: Galvanization b) Cathodic coatings: Cathodic coatings involve coating a base metal with more noble metals, which are cathodic to the base metal. Metals such as Copper, Nickel, Tin, and Silver etc are coated on Iron. One of the disadvantage of Cathodic coatings is if coating ruptures it leads more corrosion because of small anodic area and large cathodic area. Example: Tinning

Surface Conversion Coatings

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Surface conversion coatings are chemical conversion coatings. The surface layer of the base metal is converted into a compound by chemical or electrochemical reactions, which prevents the base metal form corrosion. The coating can be done by chemical dip, spray or by electrolytic method. The coating helps in the increased electrical insulation, enhanced adherence for paints and prevention of corrosion. Example: Anodizing , Phosphating

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Explain the Process of Galvanization

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It is a process of coating the base metal surface with Zinc, tin, lead, or aluminium metal. Example: coating Zinc on Iron by hot dipping Method. It involves the following steps. • The Iron metal surface is washed with organic solvents to remove oil, grease etc content on the metal surface. • Then the metal is passed through dilute sulphuric acid to remove rust and other depositions. Finally the metal is washed with water and dried. • The metal is then dipped in molten Zinc and passed through Ammonium chloride and Zinc chloride flux to prevent oxidation of Zinc. The excess Zinc is removed by passing through the rollers or by wiping. • Uses: Galvanisation is used for roofing sheets, buckets, bolts, nuts, nails, pipes etc.

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Explain the Process of Tinning Tinning is a process of coating the base metal with Tin (Sn). It is carried out by hot dipping method as fallows. • The base metal surface is washed with organic solvents to remove oil, grease etc content on the metal surface. • Then the metal is passed through dilute sulphuric acid to remove rust and other depositions. Finally the metal is washed with water and dried. • The metal is passed through Ammonium chloride and Zinc chloride flux and then dipped in molten Tin. Finally it is dipped in palm oil to prevent oxidation of Tin. The excess Tin is removed by passing through the rollers or by wiping. Tinning is widely used for coating steel, Cu and brass sheets which are used for making containers for storing food studs, oils, kerosene & packing food materials. Tinned Cu sheets are used for making cooking utensils & refrigeration equipments.

Electroplating or Electrodeposition

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It is probably the most important and most frequently applied industrial method of producing metallic coatings. Electroplating is carried out by a process called electrolysis. Thus in this process, the coating metal is deposited on the base metal by passing direct current through an electrolyte containing the soluble salt of the coating metal. The base metal to be electroplated is made the cathode of the electrolytic cell whereas the anode is either made of the coating metal itself or an inert material of good electrical conductivity like graphite.

Objectives: 1. To increase the resistance to corrosion. 2. To increase resistance to chemical attack. 3. To increase physical appearance and hardness. 4. To improve the surface properties. 5. To increase the decorative and commercial values of the metal.

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For electroplating of Ni, NiSO4 and NiCl2 are used as the electrolyte. For electroplating of Cr, chromic acid is used as the electrolyte. For Au plating, AuCl3 solution is taken as the electrolyte. For Cu plating CuSO4 solution is used as the electrolyte. In silver plating, AgNO3 solution is used as the electrolyte.

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Corrosion Control or Protection from Corrosion

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Cathodic Protection

In this method, the corroding metal is forced to behave like a cathode. There are two types of cathodic protection. a. Sacrificial Anodic protection on Galvanic protection In this method, the metallic structure which is to be protected from corrosion is connected to a more anodic metal by a wire so that the entire corrosion is concentrated on this more active metal. The more active metal loses electrons and get corroded and this metal is called sacrificial anode. Metals commonly employed as sacrificial anode are Mg, Zn, Al and their alloys.

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Sacrificial Anodic protection on Galvanic protection

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Applications Important applications of sacrificial anodic method include protection of buried pipe lines, underground cables, marine structures etc.

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Impressed Current Cathodic Protection

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In this method, an impressed current is applied in the opposite direction to nullify corrosion current so as to convert the corroding metal from anode to cathode. Impressed current can be derived from a direct current source like battery. An inert or insoluble electrode like graphite or silica act as anode to complete the circuit. The surroundings of anode should be filled with salts and carbon to increased the conductivity. Applications This type of cathodic protection has been applied to water coolers, water tanks, buried oil and water pipes, transmission towers etc.

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Use of Corrosion Inhibitors

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Chemicals which are added in small quantities to the corroding medium in order to reduce the corrosion rate are called corrosion inhibitors. They reduce corrosion by forming a protective film either at the cathode or anode. Thus there are two types of corrosion inhibitors – anodic inhibitors and cathodic inhibitors.

Anodic inhibitors

Chromates (CrO4), phosphate (PO4), and Tungstates (WO4) transition metals are used as anodic inhibitors. They react with the newly produced metal ions at the anode forming a protective film or barrier there by preventing further corrosion.

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Cathodic Inhibitors These are the substance, which slow down the cathodic reaction. The cathodic reactions involve liberation of hydrogen in acidic solution or OH-ions in alkaline and neutral medium. The cathodic organic inhibitors include amines, thiourea, sulphoxides etc. The two types of cathodic inhibition reactions are liberation of hydrogen, absorption of oxygen and formation of hydroxyl ions. i) Inhibition of Oxygen absorption and Hydroxyl ions

ii) Inhibition of Hydrogen liberation

Use of Corrosion Inhibitors

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Cathodic Inhibitors

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i) Inhibition of oxygen absorption and hydroxyl ions

H2O + ½ O2 + 2 e- 2 OH-

The formation of OH- ions can be prevented either by removing O2 from the medium or by decreasing the diffusion of O2 in to the cathode. O2 is removed either by adding reducing agents like Na2 SO3, N2H4 etc or by mechanical dearation.

O2 + NH2 – NH2 N2 + 2H2O 2 Na2 SO3 + O2 2Na2SO4

Salts of Zn, Mg or Ni are added to the corroding medium to reduce the diffusion of O2 towards cathode. These salts react with OH- ions at the cathode forming insoluble hydroxides which are adsorbed at the cathode.

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Cathodic Inhibitors

ii) Inhibition of Hydrogen Liberation 2H+ + 2 e- H2 (g) Evolution of H2 can be prevented by slowing down the diffusion of H+ ions to the cathode or by increasing H2 over voltage. Diffusion of H+ ions can be prevented by adding organic inhibitors such as amines, urea, thiourea etc. These are adsorbed at the surface as a film. Arsenic oxide or antimony oxide is added to increase the H2 over voltage. These oxides form adherent film of metallic arsenic or antimony at the cathodic areas.

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